This application is based on application No. 115753 filed in Japan on Apr. 17, 2000, the content of which incorporated hereinto by reference.
This invention relates to a method of manufacturing a fuse which melts and breaks due to heating with excessive current flow, and to a battery pack which contains that fuse.
A fuse shuts off current flow by melting open due to heat generated by excessive current flow. A fuse is heated by Joule heating. Joule heating increases in proportion to the square of the current times the electrical resistance of the fuse. Consequently, as current flow increases, Joule heating increases abruptly heating the fuse to high temperatures. The temperature at which a fuse blows is determined by the metal used. Thus, a fuse heats and melts open at a prescribed current flow and shuts off that current.
A battery pack contains a fuse to protect the batteries from excessive currents. A fuse is connected in series with the batteries and is melted open by excessive current. A battery pack which utilizes a lead In the dual role as a fuse has also been developed (Japanese Patent Application SHO 55-141448, 1980). The battery pack cited has a lead formed with a narrow section to melt open with excessive current.
In addition, the present inventor developed a fuse using laminates, such as bi-metal and tri-metal laminates of metals with different thermal expansion rates, to improve the breaking characteristics of a fuse with the previously cited structure (Japanese Patent Application HEI 11-154714, 1999). When this fuse is heated by excessive current, mechanical deformation stress, due to differences in thermal expansion rates of the laminated metals, acts on the section which blows. Consequently, this fuse has the characteristic of blowing quickly as a result of mechanical deformation stress acting on the fuse when heated by excessive current.
However, a fuse of this structure has the drawback of high materials cost because of the use of bi-metals or tri-metals. Further, in a fuse of this type with the structure shown in FIG. 1, the current at which the fuse blows is determined by the cross sectional area of a narrow section. Therefore, it is necessary to cut this section significantly thin. This is because the shut down current for a battery pack is small at only a few amperes.
A fuse made with a narrowly cut section has the drawback that it can be easily broken by mechanical shock, for example by dropping during battery pack assembly or after being assembled in a battery pack. If the fuse mechanically breaks, the battery pack becomes completely useless after that. Consequently, although it is important for a fuse installed in a battery pack to reliably melt open with excessive current flow, it is also important for the fuse not to break open as a result of mechanical shock.
Further, for a fuse with a locally narrow section as shown in FIG. 1, there are limitations to the width of the narrow region imposed by manufacturing practicality. For example, it is extremely difficult to fabricate this type of fuse for a fuse blowing current on the order of 3A. Therefore, this type of fuse has the drawback that it cannot be installed in a low capacity battery pack. Since the width of the narrow section must be made still narrower as the fuse blowing current becomes smaller, this type of low current fuse has the further drawback that it is becomes easier to break by mechanical shock.
The present invention was developed to resolve these types of drawbacks. Thus it is a primary object of the present invention to provide a method of fuse manufacture and a battery pack containing that fuse wherein the fuse can be inexpensively manufactured in quantity, is blown reliably with excessive current, is difficult to break with mechanical shock, and in particular, can be made resistant to mechanical shock for low fuse blowing currents.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
The present invention manufactures a fuse by a step to fabricate a metal plate with an overall long narrow shape, with a plurality of connecting pieces having separated tips and connected aft ends, and with the tip of each connecting piece arranged in a straight line; a step to connect the tips of adjacent connecting pieces by welding wire to the connecting piece tip regions which are arranged in a straight line; a step to cut the wire between each pair of two adjacent connecting pieces and to detach each individual connecting piece; and a step to bend the wire connecting each cut pair of connecting pieces, attached at both sides of the wire, in a direction which separates both ends of the connecting pieces.
The method above can inexpensively manufacture fuses in quantity, can produce a fuse that blows reliably with excessive current but is difficult to break with mechanical shock, and in particular, the method has the characteristic that a fuse can be made to blow at low currents and be strong with respect to mechanical shock. This is because the method of manufacture above has a step to fabricate a metal plate with a plurality of connecting pieces having separated tips and connected aft ends, a step to connect the tips of adjacent connecting pieces by welding wire to the connecting piece tip regions, a step to cut the wire between each pair of two adjacent connecting pieces and to detach each individual connecting piece, and a step to bend the wire connecting each cut pair of connecting pieces in a direction which separates both ends of the connecting pieces.
Further, the method above can manufacture with extreme simplicity a fuse structure having connecting pieces at both ends of a wire, and moreover can efficiently manufacture these fuses in quantity. Consequently, fuse manufacturing cost can be reduced, and they can be inexpensively manufactured in quantity.
Further, since a fuse manufactured by the method above is configured with wire as the narrow melting and breaking region, it is stronger with respect to mechanical shock than a fuse of the type shown in FIG. 1. This is because the metal wire material itself is strong, and in addition the wire can resiliently deform. Further, since the wire is bent, this bent region can absorb and dissipate mechanical shock. In particular, since a folded back configuration for connecting piece tips allows both ends of the wire to be swaged in the folded back tips and welded to the connecting pieces, these attachment points have significant strength and their separation is difficult. Additionally, a fuse with metal wire as the narrow fusing element has a uniform cross sectional area. Since the cross sectional area of a given metal wire determines its resistance, the characteristic of reliable fuse blowing at the specified current, particularly for small currents, is achieved.
Preferably, in the method described above, the wire is bent to align the pair of connecting pieces in a straight or nearly straight line.
Connecting pieces, which connect to the fuse wire, preferably have additional processing to provide a fold back at their tips. Wire is inserted in a folded back tip, pressure is applied to swage the wire in the folded back tip, and the swaged wire is welded. A connecting piece of this structure can form a strong and reliable connection with wire via the folded back tip. Additionally, It is preferable to use SUS-304 stainless steel wire as the fuse wire,
Still further, the metal plate of the present invention can be formed as a long narrow band and wire can be welded to connecting piece tip regions as metal plate is continuously supplied. For example, this method of manufacture can continuously produce fuses via an automated manufacturing line. Therefore, manufacturing efficiency can be improved and low cost production in quantity is possible.
The battery pack of the present invention is provided with a plurality interconnected rechargeable batteries disposed in parallel positions, and a fuse connected at both ends to a rechargeable battery electrode to form a series connection with the rechargeable batteries. The fuse is provided with a metal wire which melts open with a prescribed current flow, and a pair of connecting pieces which connect to both ends of the wire and are welded to battery electrodes. The tip region of a connecting piece is bent to provide a folded back tip, and wire is welded to the connecting piece while inserted in this folded back tip. Furthermore, wire welded at the folded back tips is bent in a direction which separates both ends of the connecting pieces attached to both ends of the wire, and the pair of connecting pieces is joined to battery electrodes.
A battery pack structured as described above uses a fuse with a low blow-out current which is also strong with respect to mechanical shock. Therefore, the fuse can reliably blow when excessive current flows, but it will not break due to mechanical shock making high reliability and quality possible.